Optical pickup apparatuses for detecting a focus-error signal by a spot size detection method (SSD method) using diffracted light caused by a diffraction element are known. Such an optical pickup apparatus is described with reference to FIGS. 6 through 8.
FIG. 6 illustrates an example structure of an optical pickup apparatus for detecting a focus-error signal by an SSD method.
A laser beam emitted from a laser light source 21 such as a laser diode reaches an objective lens 23 through a beam splitter 22, and is irradiated via the objective lens 23 onto an information recording surface of an optical recording medium 10 such as an optical disc.
The light reflected from the optical recording medium 10 returns to the beam splitter 22 through the objective lens 23, the optical path thereof being refracted by the beam splitter 22, and is directed to a diffraction element 24.
The reflected light is divided by the diffraction element 24 into the 0 order light passing therethrough, and +1 order light (diffracted light) and −1 order light (diffracted light) diffracted by the diffraction element 24.
The 0 order light, the +1 order light, and the −1 order light reach a light-receiving device section 25.
In the light-receiving device section 25, for example, light-receiving patterns shown in FIG. 8 are formed.
A light-receiving device 31 has a light-receiving region E corresponding to the 0 order light.
A light-receiving device 32 has three divided light-receiving regions A, S1, and B, and corresponds to the +1 order light.
A light-receiving device 33 also has three divided light-receiving regions C, S2, and D, and corresponds to the −1 order light.
Each of the light-receiving regions E, A, S1, B, C, S2, and D of the light-receiving devices 31, 32, and 33 outputs an electrical signal having a current level corresponding to the light intensity of the incident light.
The electrical signal output from each of the light-receiving devices 31, 32, and 33 is supplied to a matrix amp (not shown) for processing, such as current-to-voltage conversion, amplification, and matrix calculation, thereby generating a required signal.
That is, a playback signal, focus-error signal, tracking error signal, etc., corresponding to the information recorded in the optical recording medium 10 are generated.
The objective lens 23 is held by a two-axis mechanism (not shown) having a focus coil and a tracking coil so as to be displaceable in the near-and-apart direction with respect to the optical recording medium 10 (focusing direction) and in the direction transverse to the track orientation of the optical recording medium (tracking direction).
A focus drive signal is generated by a servo circuit (not shown) based on the focus-error signal to drive the focus coil of the two-axis mechanism, so that the objective lens 23 is driven in the focusing direction so as to be focused with respect to the optical recording medium 10.
A tracking drive signal is further generated by the servo circuit based on the tracking error signal to drive the tracking coil of the two-axis mechanism, so that the objective lens 23 is driven in the tracking direction so as to track with respect to the optical recording medium 10.
In the SSD method, the focus-error signal is generated according to the spot size of the diffracted light.
In the focused state shown in FIG. 8(a), the spot size of the +1 order light incident on the light-receiving device 32 is equivalent to the spot size of the −1 order light incident on the light-receiving device 33.
On the other hand, in the defocused state where the objective lens 23 is too close to or too far from the optical recording medium 10, as shown in FIGS. 8(b) and 8(c), the spot size of the +1 order light incident on the light-receiving device 32 is different from the spot size of the −1 order light incident on the light-receiving device 33.
Accordingly, by comparing the spot sizes on the light-receiving devices 32 and 33, the focus-error signal can be generated.
More specifically, the focus-error signal is generated by, in the subsequent matrix amp, calculating (A+B+S2)−(C+D+S1) on the outputs of the light-receiving regions A, S1, B, C, S2, and D.
In general, when the objective lens 23 moves from the position most distant from the optical recording medium 10 to the position closest thereto, as known in the art, in the focus-error signal, a so-called S-shaped curve shown in FIG. 7 is observed in the vicinity of the focused position.
A substantially linear region from peak P1 to peak P2 in the curve corresponds to a so-called in-focus region. In basic operation, when the objective lens 23 is positioned within the in-focus region, a focus servo controls the position of the objective lens 23 to be brought to the position of the origin of the S-shaped curve (i.e., the position where focus error=0) based on the focus-error signal.
As shown in FIG. 7, it is assumed herein that the distance of the in-focus region of the S-shaped signal is indicated by d. In other words, “d” is defined as the displacement distance of the optical recording medium (the distance by which the optical recording medium changes with respect to the position of the objective lens) when the S-shaped signal varies from the peak P1 to the peak P2.
Furthermore, one-side in-focus regions d1 and d2 of the S-shaped curve with respect to the origin of the S-shaped curve are defined as the displacement distances of the optical recording medium when the S-shaped signal goes from the origin of the S-shaped curve to the peaks P1 and P2 of the S-shaped curve, respectively. Then, the following equation holds true:d=d1+d2  Formula (1)
The origin of the S-shaped curve coincides with the focal position on the optical recording medium.
This relationship is established, in the standard SSD method, when the diffracted light (the +1 order light and the −1 order light) diffracted by the diffraction element 24 has the same spot diameter r, as shown in FIG. 8(a), resulting in substantial coincidence with the focal position of the 0 order light (strictly speaking, however, it is shifted towards the diffraction element 24 by L·cosθ, where θ denotes the angle of diffraction and L denotes the distance between the diffraction element 24 and the light-receiving device section 25).
It is assumed herein that the NA (numerical aperture) of the objective lens 23 is indicated by NA[L]. It is further assumed that the NA of the 0 order light in the light focused at the light-receiving device section 25 which passes through the diffraction element 24 is indicated by NA[0]. It is still further assumed that the NAs of the +1 order light and the −1 order light diffracted by the diffraction element 24 are indicated by NA[+1]′ and NA[−1]′, respectively.
It is also assumed that the NAs are so small that the following approximation applies: NA=sin θ=tan θ=θ.
Then, the following relationship is obtained:NA[−1]′<NA[+1]′  Formula (2)Thus, the following relationship holds true:NA[−1]′+NA[+1]′=2·NA[L]  Formula (3)
As shown in FIG. 6, the distances from the position of the light-receiving device section 25 (the focal position of the 0 order light) to the focal positions of the diffracted light (the +1 order light and the −1 order light) diffracted by the diffraction element 24 are indicated by D11 and D12, respectively.
Then, the above-noted one-side in-focus regions d1 and d2 of the S-shaped curve can be approximated as follows:d1={(½)·D11·(NA[+1]′)2}/(NA[L])2  Formula (4)d2={(½)·D12·(NA[−1]′)2}/(NA[L])2  Formula (5)
Since the spot diameters r of the diffracted light on the light-receiving devices 32 and 33 on the origin of the S-shaped curve are the same, the following equation is obtained:
                                                                        r                ⁢                                  /                                ⁢                2                            =                            ⁢                              D11                ·                                                      NA                    ⁡                                          [                                              +                        1                                            ]                                                        ′                                                                                                        =                            ⁢                              D12                ·                                                      NA                    ⁡                                          [                                              -                        1                                            ]                                                        ′                                                                                        Formula        ⁢                                  ⁢                  (          6          )                    Therefore, the following relationship holds true from Formula (6):D11/D12=NA[−1]′/NA[+1]′  Formula (7)
If NA[0]=NA[+1]′=NA[−1]′ can be approximated, D11 is equal to D12, and the following equation is obtained from Formulas (4) and (5):d1/d2=1  Formula (8)
Thus, the following relationship is obtained between the diffracted light (the +1 order light and the −1 order light) and the 0 order light:NA[+1]′=L/(L−D11)·NA[0]  Formula (9)NA[−1]′=L/(L+D12)·NA[0]  Formula (10)
If the distance L between the diffraction element 24 and the light-receiving device section 25 is sufficiently large, or if the distances D11 and D12 are sufficiently small, Formula (8) holds true.
If the above-noted approximation does not apply, however, the relationship NA[0]=NA[+1]′=NA[−1]′ does not hold true, and Formula (11) rather than Formula (8) is obtained:
                                                                        d1                /                d2                            =                            ⁢                                                                                          (                                                                        NA                          ⁡                                                      [                                                          +                              1                                                        ]                                                                          ′                                            )                                        2                                    /                                                            (                                                                        NA                          ⁡                                                      [                                                          -                              1                                                        ]                                                                          ′                                            )                                        2                                                  ·                                  (                                      D11                    ⁢                                          /                                        ⁢                    D12                                    )                                                                                                        =                            ⁢                                                                    NA                    ⁡                                          [                                              +                        1                                            ]                                                        ′                                /                                                      NA                    ⁡                                          [                                              -                        1                                            ]                                                        ′                                                                                        Formula        ⁢                                  ⁢                  (          11          )                    
In this case, an asymmetric in-focus region of the S-shaped curve is exhibited. That is, the in-focus region shown in FIG. 7 is exhibited.
An asymmetric S-shaped curve means instability in gain of a focus servo signal or an asymmetric focus margin, and is disadvantageous in view of the stability in recording to and playback from an optical recording medium.
In a device supporting a high-density recording medium, the objective lens 23 has a high NA. In order to accomplish the same in-focus region d of the S-shaped curve as that described above, as is understood from Formulas 4 and 5, the focal change distance D11 (D12) with respect to the diffraction element 24 must increase as the numerical aperture (NA[L]) of the objective lens 23 increases.
This further results in a greater amount of change in the NA of the diffracted light diffracted by the diffraction element 24 than that of the 0 order light, as is given by Formulas 9 and 10.
In an optical pickup apparatus which includes the objective lens 23 having a high NA, therefore, a more asymmetric in-focus region of the S-shaped curve is exhibited.
Furthermore, desirably, the distance L from the diffraction element 24 to the light-receiving device section 25 should be reduced in order to reduce the size of the optical pickup apparatus.
However, as is understood from Formulas 9 and 10, as the distance L from the diffraction element 24 to the light-receiving device section 25 becomes shorter, the amount of change in the NA of the diffracted light diffracted by the diffraction element 24 becomes greater than that of the 0 order light. Thus, this case also results in a more asymmetric in-focus region of the S-shaped curve.
As described above, an optical pickup apparatus which obtains a focus-error signal by the SSD method using a diffraction element has a problem of such an asymmetric in-focus region of the S-shaped curve.
A noticeably asymmetric in-focus region of the S-shaped curve is exhibited, resulting in a large problem, particularly in an optical pickup apparatus which includes the objective lens 23 having a high NA and in which the distance L from the diffraction element 24 to the light-receiving device section 25 is small, that is, a compact optical pickup apparatus used for high-density optical recording media.